Improve Json constructor.

tests/lib/convert/json_toEncodable_reviver_test.dart

+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library json_tests;
+import 'package:expect/expect.dart';
+import 'dart:convert';
+
+class A {
+  final x;
+  A(this.x);
+}
+
+toEncodable(A a) => { "A": a.x };
+reviver(key, value) {
+  if (value is Map && value.length == 1 && value["A"] != null) {
+    return new A(value["A"]);
+  }
+  return value;
+}
+
+const extendedJson =
+    const JsonCodec(toEncodable: toEncodable, reviver: reviver);
+
+main() {
+  var encoded = extendedJson.encode([new A(0), { "2": new A(1) }]);
+  Expect.equals('[{"A":0},{"2":{"A":1}}]', encoded);
+  var decoded = extendedJson.decode(encoded);
+  Expect.isTrue(decoded is List);
+  Expect.equals(2, decoded.length);
+  Expect.isTrue(decoded[0] is A);
+  Expect.equals(0, decoded[0].x);
+  Expect.isTrue(decoded[1] is Map);
+  Expect.isNotNull(decoded[1]["2"]);
+  Expect.isTrue(decoded[1]["2"] is A);
+  Expect.equals(1, decoded[1]["2"].x);
+
+  var a = extendedJson.decode(extendedJson.encode(new A(499)));
+  Expect.isTrue(a is A);
+  Expect.equals(499, a.x);
+}